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http://inchoo.net/magento/how-to-make-configurable-options-autoselected-on-configurable-product-view-page/

/app/design/frontend/default/your_theme/template/catalog/product/view/type/options/configurable.phtml

<script type="text/javascript">
  var spConfig = new Product.Config(<?php echo $this->getJsonConfig() ?>);
  
  //we create new function
  spConfig.setInitialState = function(dropdown_id)
  {
   
      //select dropdown
      var dropdown = $(dropdown_id);
      
      //remove empty option from dropdown so it is not selectable after initial selection
      dropdown[0].remove();
      
      //change selections in dropdowns
      for(index = 0; index < dropdown.length; index++)
      { 
          if(dropdown[index].value != "")
          {
              dropdown.selectedIndex = index;
               
              var element = dropdown;
              var event = 'change';
               
              //fire events
              if(document.createEventObject)
              {
                  var evt = document.createEventObject();
                  return element.fireEvent('on'+event,evt)
              }
              else
              {
                  var evt = document.createEvent("HTMLEvents");
                  evt.initEvent(event, true, true );
                  return !element.dispatchEvent(evt);
              }
          }
      }
  };
  
  <?php foreach($_attributes as $_attribute): ?>
      spConfig.setInitialState("attribute<?php echo $_attribute->getAttributeId() ?>")
  <?php endforeach; ?>
</script>